This invention relates to optical fiber communication systems and, in particular to a filter for compensating polarization mode dispersion in such systems.
Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a light source, a modulator for impressing information on the light, an optical fiber transmission line for carrying the optical signal and a receiver for detecting the signals and demodulating the information they carry. Increasingly the optical signals are wavelength division multiplexed (xe2x80x9cWDMxe2x80x9d) signals, comprising a plurality of distinct wavelength signal channels.
Polarization mode dispersion (xe2x80x9cPMDxe2x80x9d) is a significant problem in high bit rate transmission over long distances in optical fiber. PMD is caused by small irregularities in the shape or stress of the optical fiber core. Distortion created by irregularities in the shape of the optical fiber cable and its core. Stress or core asymmetries cause the incoming light to travel at a different speed depending on the orientation of its electric field, or polarization. Since the fiber does not preserve the polarization, an incoming pulse will suffer dispersion due to traveling in different polarization states and coupling between polarization states. The consequence is that the two portions arrive at slightly different times (picoseconds apart), distorting the pulse and increasing the bit error rate (xe2x80x9cBERxe2x80x9d). PMD thus limits the bit rate and transmission distance in optical fiber.
Accordingly there is a need to provide compensation for PMD in a high-speed optical fiber.
In accordance with the invention, a polarization mode dispersion compensator splits an optical signal into two optical signals. A first optical path provides polarization rotation of one of the two optical signals. The two optical signals are filtered by a combination of allpass filters and at least one 2xc3x972 coupler. A second optical path then provides polarization rotation of the other of the two optical signals before the two optical signals are combined.